Advances in cross-national comparisons of cognitive ageing
Authors: Takeshi Nakagawa
Journal: The Lancet Healthy Longevity (2023)

As the global population continues to age, the number of people with dementia is estimated to increase considerably over the next three decades, especially in low-income and middle-income countries.1Nichols E Steinmetz JD Vollset SE et al.Estimation of the global prevalence of dementia in 2019 and forecasted prevalence in 2050: an analysis for the Global Burden of Disease Study 2019.Lancet Public Health. 2022; 7: e105-e125Summary Full Text Full Text PDF PubMed Scopus (775) Google Scholar This projection highlights the importance of research to identify both risk and protective factors of cognitive ageing across countries and to translate evidence into policy and clinical practice. In The Lancet Healthy Longevity, Alden L Gross and colleagues2Gross AL Li C Briceño EM et al.Harmonisation of later-life cognitive function across national contexts: results from the Harmonized Cognitive Assessment Protocols.Lancet Healthy Longev. 2023; 4: e573-e583Google Scholar report on cognitive function scores that are comparable across countries, generated on the basis of a battery of neuropsychological tests under the Harmonized Cognitive Assessment Protocol (HCAP). The study team harmonised publicly available datasets from six studies: the Health and Retirement Study (HRS) in the USA and HRS partner studies in China, England, India, Mexico, and South Africa. Their work brings the novelty of a statistical harmonisation approach via an item banking procedure and confirmatory factor analysis with use of graded-response item response theory models. By contrast, previous research has adopted a prestatistical, methodological harmonisation approach (ie, coordinated analysis) by equating tests and analytical models across studies—for example, Piccinin and colleagues3Piccinin AM Muniz-Terrera G Clouston S et al.Coordinated analysis of age, sex, and education effects on change in MMSE scores.J Gerontol B Psychol Sci Soc Sci. 2013; 68: 374-390Crossref PubMed Scopus (91) Google Scholar compared the trajectories of Mini-Mental State Examination scores across six studies in four countries. The HRS and its partner studies also adopted a methodological harmonisation approach and provided data on cognitive tests that were common across countries. In addition, Gross and colleagues’ datasets,2Gross AL Li C Briceño EM et al.Harmonisation of later-life cognitive function across national contexts: results from the Harmonized Cognitive Assessment Protocols.Lancet Healthy Longev. 2023; 4: e573-e583Google Scholar based on the statistical harmonisation approach, used all available data on cognitive tests in the HCAPs. As noted elsewhere,4Torres JM Glymour MM Future directions for the HRS Harmonized Cognitive Assessment Protocol.Forum Health Econ Policy. 2022; 25: 7-27Crossref Scopus (1) Google Scholar questions about HCAPs remain. First, the measurement equivalence of HCAPs (ie, whether tests or measures of interest can be compared across different groups) requires further investigation across contexts (eg, racial and ethnic groups, cultures, and languages). Gross and colleagues2Gross AL Li C Briceño EM et al.Harmonisation of later-life cognitive function across national contexts: results from the Harmonized Cognitive Assessment Protocols.Lancet Healthy Longev. 2023; 4: e573-e583Google Scholar tested whether or not a factor structure consisting of general and domain-specific factors (ie, general cognitive function and orientation, memory, executive function, and language), factor loadings, and item thresholds or intercepts were equivalent across countries. However, the confirmatory factor analysis models suggested that some cognitive tests were not strictly comparable. In particular, cognitive tests to measure the language domain could function differently in Mandarin and in English as a consequence of different social and cultural backgrounds, more than due to differences in the language itself. The original cognitive tests5Langa KM Ryan LH McCammon RJ et al.The Health and Retirement Study Harmonized Cognitive Assessment Protocol Project: study design and methods.Neuroepidemiology. 2020; 54: 64-74Crossref PubMed Scopus (61) Google Scholar were developed in a high-income country (ie, the USA).6The World BankThe world by income and region.https://datatopics.worldbank.org/world-development-indicators/the-world-by-income-and-region.htmlDate: 2022Date accessed: September 12, 2023Google Scholar Thus, further considerations are necessary for the design of context-free and context-specific cognitive tests to generate cognitive function scores that are comparable across contexts. This same author group has pursued this line of research in the USA and Mexico.7Briceño EM Arce Rentería M Gross AL et al.A cultural neuropsychological approach to harmonization of cognitive data across culturally and linguistically diverse older adult populations.Neuropsychology. 2023; 37: 247-257Crossref PubMed Scopus (4) Google Scholar Second, future research should test the clinical validity of the HCAP and define cutoff scores for screening dementia across countries. By doing so, researchers can estimate the prevalence and incidence of dementia within and across countries. A previous study8Crimmins EM Kim JK Langa KM Weir DR Assessment of cognition using surveys and neuropsychological assessment: the Health and Retirement Study and the Aging, Demographics, and Memory Study.J Gerontol B Psychol Sci Soc Sci. 2011; 66: i162-i171Crossref PubMed Google Scholar has defined a cutoff score for the diagnosis of dementia on the basis of data from the Aging, Demographics, and Memory Study (ADAMS),9Langa KM Plassman BL Wallace RB et al.The Aging, Demographics, and Memory Study: study design and methods.Neuroepidemiology. 2005; 25: 181-191Crossref PubMed Scopus (274) Google Scholar in which a clinical diagnosis of dementia was assessed in a subsample of HRS respondents. More of these substudies would be useful to provide important insights for practice and policy within and across countries, although they tend to be resource-intensive and can be challenging to conduct, especially in low-income countries. Nevertheless, efforts should be made for the HCAP to continue to be adapted to and implemented in low-income and medium-income countries. Although the global network of cross-national HRS partner studies is increasing, scientific data on ageing outside of high-income countries remains relatively scarce. Thus, effort and collaboration are needed to continually update and release cognitive function scores generated on the basis of the HCAP. These endeavours might enable better cross-national comparisons of cognitive ageing across broad contexts. Using a statistical harmonisation approach, Gross and colleagues2Gross AL Li C Briceño EM et al.Harmonisation of later-life cognitive function across national contexts: results from the Harmonized Cognitive Assessment Protocols.Lancet Healthy Longev. 2023; 4: e573-e583Google Scholar have generated cognitive function scores on the basis of HCAP tests that are comparable across six countries (ie, China, England, India, Mexico, South Africa, and the USA). Researchers worldwide can access the HCAP data and support cross-national comparisons of cognitive ageing, and several projects have already used data from the HRS and its partner studies.10Lee J Phillips D Wilkens J Gateway to global aging data: resources for cross-national comparisons of family, social environment, and healthy aging.J Gerontol B Psychol Sci Soc Sci. 2021; 76: S5-16Crossref PubMed Scopus (13) Google Scholar As data from more HCAPs are released from different countries, cross-national research on cognitive ageing worldwide can be expected to flourish. However, given potential measurement biases, the direct comparisons of HCAP scores across countries should be interpreted with caution, and risk and protective factors of cognitive ageing must continue to be investigated. I declare no competing interests. Harmonisation of later-life cognitive function across national contexts: results from the Harmonized Cognitive Assessment ProtocolsWe statistically harmonised cognitive function measures across six large population-based studies of cognitive ageing. These harmonised cognitive function scores empirically reflect comparable domains of cognitive function among older adults across the six countries, have high reliability, and are useful for population-based research. This work provides a foundation for international networks of researchers to make improved inferences and direct comparisons of cross-national associations of risk factors for cognitive outcomes in pooled analyses. Full-Text PDF Open Access